Recent years have seen increasing high performance in electronic devices such as mobile information devices and information appliances following the development of digital technology. With the increased high performance in these electronic devices, miniaturization and increase in speed of semiconductor elements used are rapidly advancing. Among these, application for large-capacity nonvolatile memories represented by a flash memory is rapidly expanding. In addition, as a next-generation new-type nonvolatile memory to replace the flash memory, research and development on a variable resistance nonvolatile storage element which uses what is called a variable resistance element is advancing. Here, variable resistance element refers to an element having a property in which a resistance value reversibly changes according to electrical signals, and capable of storing information corresponding to the resistance value in a nonvolatile manner.
As an example of such variable resistance element, there is proposed a nonvolatile memory device having a variable resistance layer in which transition metal oxides of different oxygen content are stacked. For example, Patent Literature 1 discloses selectively causing the occurrence of oxidation/reduction reaction in an electrode interface which is in contact with a variable resistance layer having high oxygen content, to stabilize resistance change.
The aforementioned conventional variable resistance element includes a lower electrode, a variable resistance layer, and an upper electrode, and a memory array is configured from a two-dimensional or three-dimensional array of such variable resistance element. In each of the variable resistance elements, the variable resistance layer is of a stacked structure including a first variable resistance layer and a second variable resistance layer, and, in addition, the first and second variable resistance layers comprise the same type of transitional metal oxide. The oxygen content of the transitional metal oxide comprised in the second variable resistance layer is higher than the oxygen content of the transitional metal oxide comprised in the first variable resistance layer. By adopting such a structure, when voltage is applied to the variable resistance element, most of the voltage is applied to the second variable resistance layer which has higher oxygen content and exhibits a higher resistance value. Furthermore, oxygen, which can contribute to the reaction, is abundant in the vicinity of the interface. Therefore, oxidation/reduction reaction occurs selectively at the interface between the upper electrode and the second variable resistance layer, and stable resistance change can be realized.